1. Field of the Invention
The present invention relates to a solid electrolytic capacitor including an anode of valve action metal, a dielectric layer, a solid electrolytic layer, and a cathode conductive layer, as well as a method of manufacturing such a solid electrolytic capacitor.
2. Description of Related Art
An example of a solid electrolytic capacitor is disclosed in Japanese Unexamined Patent Application Publication No. Hei-05-159987. This capacitor consists of a capacitor element having an anode made of valve action metal. On the surface of the anode, the capacitor element sequentially includes a dielectric layer that is an oxide of the valve action metal, a solid electrolytic layer made of conductive polymer, a conductive carbon layer made of, for example, graphite, and a conductive layer made by hardening and drying conductive paste. The capacitor element is connected to a cathode terminal with a conductive adhesive. With the cathode terminal partly protruding outside, the capacitor element and cathode terminal are sealed with a casing made of, for example, epoxy resin.
Solid electrolytic capacitors of this type are widely used in power sources of CPUs and peripheral circuits of electronic appliances such as notebook computers, desktop computers, cellular phones, portable information terminals, digital cameras, and video information devices. In a power source, the solid electrolytic capacitor is used to smooth source power and remove high-frequency noise. In a switching regulator that drives a switching element at frequencies of several tens to hundreds of kilohertz, the solid electrolytic capacitor is used to remove high-frequency noise produced by the switching element. Since conductive polymers have been used for the solid electrolytic layers of the solid electrolytic capacitors, the capacitors have become smaller in size, lower in profile, and higher in capacity. Due to this, the solid electrolytic capacitors are widely used in the field of electronic information devices.
The solid electrolytic capacitors used for electronic information devices are required to reduce resistance components as small as possible. The resistance components to be reduced include 1/ωC (ω=2 πf, where f is a frequency), ESR (equivalent series resistance), and ESL (equivalent series inductance). Among them, ESR plays a major role in adverse affect in a high-frequency region. ESR is a total of resistance values of the anode, solid electrolytic layer, conductive carbon layer, conductive paste layer, and conductive adhesive layer, and the like in the solid electrolytic capacitor. Minimizing ESR is important for the solid electrolytic capacitors, in particular, for those used in high-frequency regions.
To reduce the ESR of solid electrolytic capacitors, various techniques have been developed. The inventor of the present invention has found that a factor of increasing the ESR of a solid electrolytic capacitor is a two-layer structure employed for a cathode conductive layer that connects a solid electrolytic layer to a cathode terminal. The cathode conductive layer of two-layer structure is made of a conductive paste layer and a conductive adhesive layer, and an interface between the two layers produces contact resistance that increases ESR in high-frequency regions.